Seat weight sensor having fluid filled bladder

ABSTRACT

A seat weight sensor incorporates a fluid containing bladder placed in series with the load path in the seat, whereby a load applied to and distributed across the bladder increases the pressure of the fluid therein. The pressure of the fluid is measured by a pressure sensor and is substantially proportional to the magnitude of the applied load, and substantially inversely proportional to the supported area of the bladder. The output signal is substantially linear with respect to weight. Preferably, the amount of fluid in the bladder should be less than the capacity of the bladder when the bladder is unloaded. The seat weight sensor is incorporated into an occupant restraint system for controlling the safety restraint system responsive to the weight of the occupant.

CROSS-REFERENCE TO RELATED APPLICATIONS

The instant application claims the benefit of prior U.S. ProvisionalApplication Ser. No. 60/032,380 filed Dec. 19, 1996.

TECHNICAL ART

The instant invention generally relates to sensors and systems formeasuring weight and more particularly to a weight sensor for measuringthe weight of occupants and other objects in a motor vehicle seat suchas useful determining occupant seating conditions for controlling avehcile safety restraint sytsem.

BACKGROUND OF THE INVENTION

A vehicle may contain automatic safety restraint actuators that areactivated responsive to a vehicle crash for purposes of mitigatingoccupant injury. Examples of such restraint actuators include air bags,seat belt pretensioners, and deployable knee bolsters.

One objective of an automatic safety restraint system is to mitigateoccupant injury, thereby not causing more injury with the automaticrestraint system than would be caused by the crash had the automaticrestraint system not been activated. Notwithstanding the protectivebenefit of these automatic safety restraint actuators, there isgenerally both a risk and a cost associated with the deployment thereof.Generally, it is desirable to only activate automatic safety restraintactuators when needed to mitigate injury because of the expense ofreplacing the associated components of the safety restraint system, andbecause of the potential for such activations to harm occupants. This isparticularly true of air bag restraint systems, wherein occupants tooclose to the air bag at the time of deployment--i.e. out-of-positionoccupants--are vulnerable to injury or death from the deploying air bageven when the associated vehicle crash is relatively mild. Moreover,occupants who are of small stature or with weak constitution, such aschildren, small adults or people with frail bones are particularlyvulnerable to injury induced by the air bag inflator. Furthermore,infants properly secured in a normally positioned rear facing infantseat (RFIS) in proximity to a front seat passenger-side air bag are alsovulnerable to injury or death from the deploying air bag because of theclose proximity of the infant seat's rear surface to the air baginflator module.

Air bag inflators are designed with a given restraint capacity, as forexample, the capacity to protect an unbelted normally seated fiftiethpercentile occupant when subjected to a 30 MPH barrier equivalent crash,which results in associated energy and power levels which can beinjurious to out-of-position occupants. While relatively infrequent,cases of injury or death caused by air bag inflators in crashes forwhich the occupants would have otherwise survived relatively unharmedhave provided the impetus to reduce or eliminate the potential for airbag inflators to injure the occupants which they are intended toprotect.

One technique for mitigating injury to occupants by the air bag inflatoris to reduce the power and energy levels of the associated air baginflator, for example by reducing the amount of gas generant in the airbag inflator, or the inflation rate thereof. This reduces the risk ofharm to occupants by the air bag inflator while simultaneously reducingthe restraint capacity of the air bag inflator, which places occupants agreater risk for injury when exposed to higher severity crashes.

Another technique for mitigating injury to occupants by the air baginflator is to control the rate of inflation or the capacity of theinflator responsive to a measure of the severity of the crash. However,the risk of injury to such occupants would not be mitigated under theconditions of higher crash severity when the inflator is intentionallymade aggressive in order to provide sufficient restraint for normallypositioned occupants.

Yet another technique for mitigating injury to occupants by the air baginflator is to control the activation of the air bag inflator responsiveto the presence, position, and size of the occupant, or to the severityof the crash. For example, the air bag inflator can be disabled if theoccupant weight is below a given threshold. Moreover, the inflationcapacity can be adjusted by controlling the number of inflation stagesof a multi-stage inflator that are activated. Furthermore, the inflationpower can be adjusted by controlling the time delay between the firingsof respective stages of a multi-stage inflator.

One measure of restraint capacity of an air bag inflator is the amountof occupant kinetic energy that can be absorbed by the associated airbag system, whereby when the occupant collides with the gas filled airbag, the kinetic energy of the occupant is converted to potential energyvia the pressurization of the air bag, and this potential energy isdissipated by venting pressurized gases from the air bag. As a vehiclein a crash is decelerated, the velocity of an unrestrained occupantrelative to the vehicle increases. Preferably, the occupant restraintprocess is commenced early in the crash event so as to limit the amountof occupant kinetic energy that must be absorbed and thereby minimizethe associated restraint forces and accelerations of and loads withinthe occupant. If the occupant were a simple inertial mass withoutfriction relative to the vehicle, the kinetic energy of the occupantwould be given by 1/2 M·V², where M is the mass of the occupant and V isthe occupant velocity relative to the vehicle. If a real occupant wererepresented by an interconnected set of bodies, some of which havefriction relative to the vehicle, each body of which may have differingvelocities relative the vehicle, the above equation would apply to themotion of the center of gravity of the occupant. Regardless of therepresentation, occupants of larger mass will have a larger kineticenergy for the same velocity relative to the vehicle. Therefore, anoccupant weight sensor is useful in an air bag system with variablerestraint capacity to enable the restraint capacity to be preferentiallyadapted to the weight, or mass, of the occupant.

Except for some cases of oblique or side-impact crashes, it is generallydesirable to not activate an automatic safety restraint actuator if anassociated occupant is not present because of the otherwise unnecessarycosts and inconveniences associated with the replacement of a deployedair bag inflation system. Occupant presence can be detected by a seatweight sensor adapted to provide either a continuous measure of occupantweight or to provide a binary indication if the occupant weight iseither above or below a specified weight threshold.

Known seat weight sensors comprise one or more pads employing forcesensitive resistive (FSR) films. These arrangements are typically usedas weight threshold systems to disable a passenger air bag when the seatis empty. Load cells attached to the seat mounting posts have also beenused in research applications. Mechanisms that use string basedpotentiometers to measure downward seat displacement have also beeninvestigated.

Such known arrangements suffer from several drawbacks. First, variableresistance force sensors have limited sensitivity and in some situationsare not sensitive enough to put directly under a seat pad while stillachieving the desired response. Second, the threshold weight systemprovides only very limited information. For example, such arrangementsprovide no indication as to the size of an occupant. Third, theresistance values of known variable force resistor change withtemperature, and are subject to drift over time with a constant load onthe sensor.

Furthermore, other known sensing arrangements do not otherwise providesuitable results. For example, the use of load cells is prohibitivelyexpensive for large-scale commercial applications. Strain gauges of anytype may be impractical because of the difficulty in applying them tothe strained material. Finally, mechanical string potentiometer basedweight sensors are complex, and subject to failure from stretching ofthe string.

The prior art also teaches the use of seat weight sensors outside theautomotive environment, for example as a means for disabling theactivation of either a boat or an industrial machine if the operator isnot properly seated, or for weighing a person seated on an exercisebike. These devices employ pneumatic bladders located in the seat,whereby the pressure within the bladder is used to either activate athreshold switch or to provide a continuous indication of occupantweight.

One problem with prior art pneumatic sensors, particularly when appliedto the automotive environment, is their sensitivity to environmentalconditions, particularly to ambient temperature and pressure. A seatweight sensor in an automotive environment must function reliably andaccurately over a wide range of temperatures and pressures which cancause significant errors.

The prior art also teaches the use of hydraulic load cells, wherein theweight to be measured acts upon a piston element of known area, wherebythe measured weight is found by multiplying a measured pressure timesthe known area. One problem with hydraulic load cells in the automotiveenvironment, particularly in a seat, is that the effects of load cellorientation on hydraulic head can introduce load measurement errors.

SUMMARY OF THE INVENTION

The instant invention overcomes the above-noted problems by providing aseat weight sensor which incorporates a fluid containing bladder placedin series with the load path in the seat, whereby a load applied to anddistributed across the bladder increases the pressure of the fluidtherein. The pressure of the fluid is measured by a pressure sensor andis substantially proportional to the magnitude of the applied load, andsubstantially inversely proportional to the supported area of thebladder. The instant invention also incorporates a means fordistributing the applied load across the area of the fluid containingbladder so as to prevent a concentrated load from compressing the topand bottom surfaces of the bladder against one another and therebycreating an alternate load path which does not cause an associatedpressurization of the fluid. The output signal is substantially linearwith respect to weight provided that 1) the weight is distributed over asufficient area so that the bladder does not bottom out, 2) the heightof the bladder is sufficiently small relative to the base dimensions sothat the effect of loading on the support area is relatively small.Preferably, the amount of fluid in the bladder should be less than thecapacity of the bladder when the bladder is unloaded. Otherwise, thefluid in the bladder can be pressurized by increasing temperature ordecreasing ambient pressure which results in associated load measurementerrors.

The bladder may incorporate either a liquid or a gas as the sensingfluid. A gaseous sensing fluid is prone to expansion and contractionresulting from changes in ambient temperature and pressure relative tothe conditions under which the bladder was initially filled. A gaseousfluid is also more prone to leakage and to localized collapse of the topand bottom surfaces of the bladder under the influence of a concentratedload. When located in the seat under a cushion, the cushion can providean effective distribution of the loads applied to the seat. A sheet ofsemi-rigid material can also be used to distribute load to the bladder,particularly the reaction forces from the seat springs if the bladder islocated thereon.

The bladder may incorporate internal seams which secure the top andbottom surfaces of the bladder to one another within the periphery ofthe bladder without disrupting the fluid communication within thebladder. These seams prevent the bladder from bulging in the center whenthe fluid expands due to temperature or pressure effects. Such bulgingis detrimental to seating comfort. The seams also assist reducing theoverall thickness of the bladder and in conserving the necessary amountof sensing fluid, which reduces cost when liquids such as silicone basedfluids are used. The internal seams are also effective for modifying thesensitivity of the bladder. For example, a bladder may be more sensitiveto central loads than to distal loads as might result when a portion ofthe applied load is carried by a portion of the seat cushion which isnot in series with the load bladder load path. In this case, selectivezones within the bladder, for example near the center, may be isolatedfrom the sensing fluid by a closed path seam such that a load appliedthereto is not sensed by the fluid within the bladder.

The bladder may be constructed from several sheets of fabric, such asnylon, coated with a sealably weldable coating, such as polyurethanewhich can be RF (radio frequency) welded. A coating can be applied tothe outside of the bladder to increase the membrane stiffness thereofand thereby facilitate the distribution of applied loads.

The instant invention integrates pressure over the entire loading areaof the seat, thereby producing a consistent output signal that isrelatively insensitive to the associated load distribution. The instantinvention is relatively flexible, and when installed under the seatcushion does not interfere with seating comfort. Furthermore, thisinstallation is relatively easy, thereby minimizing the impact on theoverall manufacturing process of the seat/vehicle.

Accordingly, one object of the instant invention is to provide animproved seat weight sensor which provides a consistent and accuratemeasure of the seat loading independent of the location of the source ofweight on the seat.

A further object of the instant invention is to provide an improved seatweight sensor which provides a consistent and accurate measure of theseat loading independent of the size and distribution of the source ofweight on the seat.

A yet further object of the instant invention is to provide an improvedseat weight sensor which provides a consistent and accurate measure ofthe seat loading independent of the amount of weight on the seat.

A yet further object of the instant invention is to provide an improvedseat weight sensor which operates under a wide range of ambienttemperature and pressure conditions.

A yet further object of the instant invention is to provide an improvedseat weight sensor which can distinguish between a rear facing infantseat, for which an air bag system is preferably not deployed, and otheroccupants for which an air bag system is preferably deployed in theevent of a crash of sufficient severity.

A yet further object of the instant invention is to provide an improvedseat weight sensor which can be incorporated into an intelligent safetyrestraint system for which the preferable mode of the activation of acontrollable occupant restraint system is dependent upon the weight ofthe occupant.

A yet further object of the instant invention is to provide an improvedseat weight sensor which does not interfere with occupant comfort.

A yet further object of the instant invention is to provide an improvedseat weight sensor which is insensitive to the orientation of the seat.

A yet further object of the instant invention is to provide an improvedseat weight sensor which is inexpensive to produce.

In accordance with these objectives, one feature of the instantinvention is a fluid containing bladder mounted in the base of the seat.

Another feature of the instant invention is a pressure sensoroperatively coupled to the fluid containing bladder for measuring thepressure therein.

Yet another feature of the instant invention is a differential pressuresensor operatively coupled to the fluid containing bladder for measuringthe pressure therein relative to local atmospheric pressure.

Yet another feature of the instant invention is the incorporation of agas as the fluid in the fluid containing bladder.

Yet another feature of the instant invention is the incorporation of aliquid as the fluid in the fluid containing bladder.

Yet another feature of the instant invention is the incorporation of ameans for distributing load across the load bearing surface of thebladder.

Yet another feature of the instant invention is that the volume of fluidin the fluid containing bladder is such that the volume of the bladderin an unloaded state is less than the maximum volume of the bladder overthe range of environmental operating conditions.

The specific features of the instant invention provide a number ofassociated advantages. One advantage of the instant invention withrespect to the prior art is that the fluid containing bladder isresponsive to loads over a large area of the seat without regards to thedistribution or amount of loading.

Another advantage of the instant invention is that the output signal isinherently relatively linear which simplifies signal analysis.

Yet another advantage of the instant invention is that the seat weightsensor thereof can enable a rear facing infant seat for which the airbag system is preferably not deployed to be distinguished from anoccupant for which the air bag system is preferably deployed.

Yet another advantage of the instant invention is that the seat weightsensor thereof is sufficiently robust, reliable and accurate to enableassociated occupant weight dependent control of a controllable occupantrestraint system.

Yet another advantage of the instant invention is that the seat weightsensor thereof is relatively inexpensive to produce.

Accordingly, the instant invention provides an improved seat weightsensor which is relatively insensitive to the effects of ambienttemperature and pressure; which is simple in construction and relativelyrobust and reliable in operation; which can be readily incorporated intoan automotive seat without interfering with occupant comfort; and whichcan be produced relatively inexpensively.

The instant invention will be more fully understood after reading thefollowing detailed description of the preferred embodiment withreference to the accompanying drawings. While this description willillustrate the application of the instant invention in an automotivesafety restraint system, it will be understood by one with ordinaryskill in the art that the instant invention can also be applied to othersystems for weighing objects.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the incorporation of the instant invention in avehicle seat.

FIG. 2 illustrates the bladder of one embodiment of the instantinvention in an unloaded condition together with several alternatepressure sensors connected to an associated signal processor whichdetermines weight from measured pressure.

FIG. 3 illustrates the instant invention responsive to one possible loaddistribution.

FIG. 4 illustrates the instant invention responsive to a second possibleload distribution.

FIG. 5 illustrates one environment of the instant invention.

FIG. 6 illustrates a second embodiment of the instant inventionconstructed from sealably interconnected sheets of flexible material.

FIG. 7 illustrates a cross section of the FIG. 6 embodiment illustratingseveral fluid containing zones within the associated bladder, and alsoillustrating an associated dead-zone.

FIG. 8 illustrates a means for distributing the support load from seatsprings across the base of the fluid containing bladder of the instantinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

Referring to FIG. 5, a seat 3 in a motor vehicle 1 incorporates ahydrostatic seat weight sensor 10 mounted in the seat base 40. Thehydrostatic seat weight sensor 10 comprises bladder 12 and adifferential pressure sensor 20 for measuring the difference in pressurebetween the bladder 12 and the atmosphere 25. The bladder 12 issandwiched between the seat frame 46 below and the seat cushion foam 44above. The bladder is filled with a fluid, either gaseous or liquid(including gels).

In operation, an occupant 5 seated on the base 40 of seat 3 causes thepressure inside a bladder 12 to increase such that product of thedifferential pressure, as sensed by differential pressure sensor 20,multiplied times the area of the base of the bladder 12 is substantiallyequal to the total weight distributed by the seat cushion foam 44 overthe top of the bladder 12. The pressure signal output 22 fromdifferential pressure sensor 20 is operatively coupled to an electroniccontrol module 50 which converts the pressure signal output 22 to ameasure of occupant weight using known analog, digital, ormicroprocessor circuitry and software. A crash sensor 60 is alsooperatively coupled to the electronic control module 50. Responsive to acrash detected by the crash sensor 60, and further responsive to thesensed weight of the occupant as transformed from the pressure signaloutput 22, the electronic control module 50 generates a signal 80 whichis operatively coupled to one or more initiators 90 of one or more gasgenerators 100 mounted in an air bag inflator module 110, therebycontrolling the activation of the air bag inflator module assembly 7 soas to inflate the air bag 120 as necessary to protect the occupant 5from injury which might otherwise be caused by the crash. The electricalpower necessary to carry out these operations is provided by a source ofpower 70, preferably the vehicle battery.

Referring to FIG. 1, the seat cushion 44 acts to distribute the loadfrom the occupant 5 across the top load bearing surface of the bladder12, thereby causing an increase in pressure of the fluid within thebladder 12 thereby supporting the top load bearing surface of thebladder 12. By distributing the load across the top load bearingsurface, the seat cushion acts to prevent concentrated loads applied tothe seat from causing the top and bottom surfaces of the bladder 12 fromcollapsing against one another, thereby creating an alternate path forload which would not cause an associated increase in pressure of thefluid.

Referring to FIG. 2, the pressure of the unloaded bladder 12 is given byP_(O), which is substantially equal to the local atmospheric pressure.The pressure in the bladder 12 may be alternately sensed by an absolutepressure sensor 20, or by one or more strain sensors 20 incorporated inor attached to the surface of the bladder 12. The signal from thepressure sensor 20 is operatively coupled to a signal processor 50 whichmeasures the weight W of the applied load therefrom.

Referring to FIG. 3, a load of weight W supported by the bladder 12causes the pressure of the fluid thereinto increase by an amount ΔP,such that the weight W is given by

    W=ΔP·A                                      (1)

where A is the effective area of the bottom load bearing surface of thebladder 12. If the bladder 12 is fully supported by the seat base 46,then the effective area A is substantially the same as the area of thebase of the bladder 12. This is true regardless of the area anddistribution of loading on the top load bearing surface of the bladder12 as is illustrated in FIG. 4, so long as the loading on the top loadbearing surface of the bladder 12 is sufficiently distributed so thatthe top surface and bottom surfaces of the bladder 12 are not collapsedupon one another within the periphery of the bladder 12.

Note that the area A of the bladder which rests upon the supportingsurface remains approximately equal regardless of the distribution ofthe weight which is applied to the bladder 12. In other words twodifferent objects each with weight, W, but with different distributionsof weight--one concentrated and one whose weight is more spreadout--will each register the same increase in pressure, ΔP. Two equalweights, each with weight W, both register the same increase inpressure. The pressure increase ΔP resulting from an applied weight isindependent of the shape of the applied weight, W, as long as thecontact area between the bottom of the bladder 12 and the supportingsurface remains constant.

The bladder 12 is preferably only partially filled with fluid with anamount such that relatively high ambient temperatures or relatively lowambient pressures do not cause the fluid of the unloaded bladder 12within the seat to become pressurized relative to local atmosphericpressure. For the bladder 12 mounted within a seat 3 as illustrated inFIG. 1, with no applied load, the pressure of the fluid within thebladder 12 will generally be higher than the local atmospheric pressureby an amount corresponding to the weight of the top surface of thebladder 12 and to the hydrostatic pressure of the fluid within thebladder 12 relative to the location of the pressure sensor 20.Typically, these incremental components of pressure are negligiblerelative to the range of loads to be measured.

The bladder 12 is preferably designed so that the contact area at thebottom load bearing surface of the bladder 12 remains relativelyconstant over 1) the expected range of applied weights, W, and weightdistributions which may result from various sizes and positions ofobjects, and 2) the expected range of ambient temperature and pressureconditions.

For situations where it is not possible to design the bladder 12 so asto prevent significant variation in the contact area at the bottom loadbearing surface of the bladder 12, then the differential pressure of thefluid within the bladder 12 may not by itself accurately indicate theapplied weight. A significant variation in the contact area will resultin an ambiguity between the increase in fluid pressure relative to theincrease in internal tension along the surface of the bladder. In thiscase, piezoresistive film can be added to the surface of the bladder inorder to measure surface tension. At the upper surface of the bladder,the weight, W, is supported by an increase in pressure, dP, as well asan increase in the surface tension of the bladder 12. When surfacetension is known, this information can be used to resolve thepressure/tension ambiguity and then accurately estimate the weight W ofthe applied load.

Referring to FIGS. 6 and 7, the bladder 12 of a hydrostatic seat weightsensor 10 is constructed from two sheets of a flexible material which iscoated with a material which can be sealably welded. The sheets offlexible material are first placed with sealably weldable coating sidesadjacent one another, and are sealed to one another by a welded seam 602along a periphery 601 so as to form an inflatable confinement. Thesheets of material are also welded to one another at a plurality ofseams 602 at locations 603 within the periphery 601 so as to form aplurality of zones in fluid communication with one another within theinflatable confinement. The bladder 12 is partially filled with a fluidwhich is distributed amongst the various zones. The amount of fluid andnumber of zones is such that the top and bottom surfaces of the bladder12 do not collapse against one another responsive to an applied load.The pressure within the bladder 12 is sensed by a pressure sensor 20attached to the outside surface of the bladder 12, whereby a change ofpressure within the bladder applies a force to a first electrode 612 ofthe pressure sensor 20 causing this electrode to deform or move relativeto a second electrode 614, thereby changing the capacitance between theelectrodes 612 and 614. The second electrode is secured to the pressuresensor housing 616 which is attached to the surface of the bladder. Arestraint 702 is located on the inside of the bladder 12 proximate thepressure sensor so as to prevent the top and bottom surfaces of thebladder 12 from collapsing against one another proximate the pressuresensor 20.

One problem which can occur with a hydrostatic seat weight sensor 10 isreduced sensitivity to loads which are distributed in the seat towardsthe periphery of the bladder. This problem can be mitigated by providinga non-uniform distribution of fluid containing zones 603 within theperiphery of the bladder. Furthermore, one or more dead zones 604 may beformed within the bladder, each by a seam 602 which defines a closedpath, whereby the portion of load applied to the seat cushion 44 in theregion of the dead zone 604 is either supported by the adjacent fluidcontaining zones, or is transferred to the seat base 46 withoutincreasing the pressure of the fluid within the bladder 12.

In an exemplary system in accordance with FIGS. 6 and 7, the top surfaceof the bladder 12 is constructed from 200 denier nylon fabric which iscoated with polyurethane, and the bottom surface of the bladder 12 isconstructed from 840 denier nylon fabric which is also coated withpolyurethane. The seams are formed by welding the polyurethane coatingsof the separate sheets together using an RF welding process. The outsideof the bottom surface is also coated with polyurethane so as todistribute localized loads across the bottom surface of the bladder 12.The bladder 12 is filled with a silicone fluid.

Referring to FIG. 6, a load distributor 802 constructed comprising sheetof semi-rigid material can be interposed between the bottom surface ofthe bladder 12 and the top of the seat suspension springs 47 so as todistribute the support loads from the seat base 46 across the bottomload bearing surface of the bladder 12 thereby preventing the top andbottom surfaces of the bladder 12 from collapsing against one anotherproximate the springs 47.

A gaseous fluid may also be incorporated into the bladder 12. Thegas-filled bladder 12 is preferably only partially filled to allow forgaseous expansion due to variations in ambient temperature and pressure,such that over the possible range of environmental operating conditionsthe volume of the unloaded gas-filled bladder 12 generally does notexceed the design capacity thereof. Moreover, under these conditions,the associated absolute pressure in the bladder would not exceed ambientpressure by more than the negligibly small amount necessary to supportthe top surface of the bladder 12.

Under the action of a distributed load, the volume of the bladder 12decreases until the pressure therein is sufficiently great to supportthe load. For a bladder 12 having a design shape of a rectangular slabhaving a height and two base dimensions, as the height decreases underthe action of the load, the base dimensions increase, thereby increasingthe base area of the bladder 12. The weight of the distributed load isthen given by the product of the base area of the bladder times thedifference in pressure inside and outside the bladder. Even if theloading on the top of the seat is relatively localized, the associatedweight is given by the differential pressure acting on the base area ofthe bladder, assuming the base of the bladder is fully supported andthat that top surface of the bladder is not locally compressed againstthe bottom surface.

For a bladder 12 with a square profile, having a height h and a basedimension S, the effects of load on the support area A of the bladder,and upon the associated differential pressure DP, relative toatmospheric pressure Patm, are illustrated in the following analyticapproximation:

From the ideal gas law, with P=absolute fluid pressure within thebladder, V=bladder volume, n=number of moles of gas, R=universal gasconstant, and Tatm the temperature of gas within the bladder,

    P·V=n·R·Tatm                    (2)

The pressure within the bladder is given by,

    P=Patm.sub.+ DP                                            (3)

Assuming the bladder is filled to a volume Vfill at a temperature Tfill,the associated pressure Pfill is given from the ideal gas law as,##EQU1##

The fill volume may also be expressed in terms of the design height h0and base dimension S0 of the bladder, and the fraction of the designvolume Vmax which is filled,

    Vmax=S0.sup.2 ·h0                                 (5)

    Vfill=α·Vmax                                (6)

The volume of the bladder, Vdpc, is then given in terms of the fillconditions, the local atmospheric condtions, and the measured pressuredifferential DP as, ##EQU2##

In this analysis, the surface area "a" of the bladder is assumed toremain constant under loading, whereby under the influence of an appliedload, the base area increases as the height decreases. This surface areais given by,

    a=2·(S.sup.2.sub.+ 2·S·h)       (8)

Solving for the base dimension S in terms of the height h and surfacearea "a" gives ##EQU3##

The base area A and volume V of the bladder are then given by, ##EQU4##

By equating equations (7) and (13) and solving for the bladder height,assuming that

    a>>2·h.sup.2                                      (14)

the height of the bladder in terms of the surface area, fill conditions,ambient conditions, and measureed pressure differential are then givenby: ##EQU5##

The measured pressure differential is related to the magnitude of theapplied load and bladder surface area by, ##EQU6##

The above equations may be solved implicitly to determine thesensitivity of the weight measurement error to the geometry of thebladder, the fill conditions, and the ambient conditions.

Generally, the sensitivity of the gas filled bladder to ambienttemperature and pressure is decreased with decreasing amounts of gas inthe bladder, and with decreasing bladder thickness (i.e. height) for thesame base dimensions of the bladder. However, as the bladder is madethinner in overall height, and the amount of gas is reduced, the bladderbecomes more susceptible to bottoming-out under the influence oflocalized loads applied to the seat.

One of ordinary skill in the art will appreciate that while the bladderof the instant invention is illustrated herein using a rectangularshape, the particular shape of the bladder is not considered to belimiting to the instant invention.

While specific embodiments have been described in detail, those withordinary skill in the art will appreciate that various modifications andalternatives to those details could be developed in light of the overallteachings of the disclosure. Accordingly, the particular arrangementsdisclosed are meant to be illustrative only and not limiting as to thescope of the invention, which is to be given the full breadth of theappended claims and any and all equivalents thereof.

We claim:
 1. A system for measuring the weight of an occupant in avehicle seat and for controlling a safety restraint system responsivethereto, comprising:a. a bladder mounted beneath the cushion of the seatand supported by the base of the seat, where in said bladder isconstructed from a flexible material; b. a fluid contained by saidbladder, the volume of said fluid in said bladder being less than thecapacity of said bladder in the absence of an external load applied tosaid bladder; c. a pressure sensor operatively coupled to said bladderfor generating a signal responsive to the pressure of said fluid withinsaid bladder; and d. a signal processor for measuring the weight of theoccupant from said signal for generating a control signal forcontrolling the safety restraint system responsive to said weightmeasurement.
 2. A system for measuring the weight of an occupant in avehicle seat and for controlling a safety restraint system responsivethereto as recited in claim 1, wherein said fluid is a gas.
 3. A systemfor measuring the weight of an occupant in a vehicle seat and forcontrolling a safety restraint system responsive thereto as recited inclaim 1, wherein said pressure sensor is responsive to the absolutepressure of said fluid within said bladder.
 4. A system for measuringthe weight of an occupant in a vehicle seat and for controlling a safetyrestraint system responsive thereto as recited in claim 1, wherein saidpressure sensor is responsive to the differential pressure of said fluidwithin said bladder relative to local atmospheric pressure.
 5. A systemfor measuring the weight of an occupant in a vehicle seat and forcontrolling a safety restraint system responsive thereto, comprising:a.a bladder mounted beneath the cushion of the seat and supported by thebase of the seat, wherein said bladder comprises a plurality of sheetsof coated fabric sealably connected to one another at a periphery so asto form an inflatable confinement within said periphery whereby saidsheets of coated fabric are further connected to one another at one ormore locations within said periphery so as to create a plurality offluid containing zones within said inflatable confinement which are influid communication with one another; b. a fluid contained by saidbladder; c. a pressure sensor operatively coupled to said bladder forgenerating a signal responsive to the pressure of said fluid within saidbladder; and d. a signal processor for measuring the weight of theoccupant from said signal for generating a control signal forcontrolling the safety restraint system responsive to said weightmeasurement.
 6. A system for measuring the weight of an occupant in avehicle seat and for controlling a safety restraint system responsivethereto as recited in claim 5, wherein said fluid containing zones arenon-uniformly distributed within said periphery.
 7. A system formeasuring the weight of an occupant in a vehicle seat and forcontrolling a safety restraint system responsive thereto as recited inclaim 6, wherein said sheets of coated fabric are further connected toone another along one or more closed paths within said periphery.
 8. Asystem for measuring the weight of an occupant in a vehicle seat and forcontrolling a safety restraint system responsive thereto as recited inclaim 5, further comprising a coating on the outside of at least oneload bearing surface of said bladder, the volume of said fluid in saidbladder being less that the capacity of said bladder in the absence ofan external load applied to said bladder, wherein said pressure sensoris responsive to the differential pressure of said fluid within saidbladder relative to local atmospheric pressure, wherein said pressuresensor is isolated from said fluid by the surface of said bladder,further comprising a restraint located on the inside of said bladderproximate said pressure sensor, whereby said restraint prevents saidbladder from collapsing proximate said pressure sensor.
 9. A system formeasuring the weight of an occupant in a vehicle seat and forcontrolling a safety restraint system responsive thereto as recited inclaim 8, wherein said fluid containing zones are non-uniformlydistributed within said periphery.
 10. A system for measuring the weightof an occupant in a vehicle seat and for controlling a safety restraintsystem responsive thereto as recited in claim 9, wherein said sheets ofcoated fabric are further connected to one another along one or moreclosed paths within said periphery.
 11. A system for measuring theweight of an occupant in a vehicle seat and for controlling a safetyrestraint system responsive thereto as recited in claim 8, wherein saidsheets of coated fabric are further connected to one another along oneor more closed paths within said periphery.
 12. A system for measuringthe weight of an occupant in a vehicle seat and for controlling a safetyrestraint system responsive thereto, comprising:a. a bladder mountedbeneath the cushion of the seat and supported by the base of the seat,wherein said bladder is constructed from a flexible material, b. a sheetof semi-rigid material proximate a load bearing surface of said bladder;c. a fluid contained by said bladder; d. a pressure sensor operativelycoupled to said bladder for generating a signal responsive to thepressure of said fluid within said bladder; and e. a signal processorfor measuring the weight of the occupant from said signal for generatinga control signal for controlling the safety restraint system responsiveto said weight measurement.
 13. A system for measuring the weight of anoccupant in a vehicle seat and for controlling a safety restraint systemresponsive thereto, comprising:a. a bladder mounted beneath the cushionof the seat and supported by the base of the seat wherein said bladderis constructed from a flexible material; b. a coating on the outside ofat least one load bearing surface of said bladder; c. a fluid containedby said bladder; d. a pressure sensor operatively coupled to saidbladder for generating a signal responsive to the pressure of said fluidwithin said bladder; and e. a signal processor for measuring the weightof the occupant from said signal for generating a control signal forcontrolling the safety restraint system responsive to said weightmeasurement.
 14. A system for measuring the weight of an occupant in avehicle seat and for controlling a safety restraint system responsivethereto, comprising:a. a bladder mounted beneath the cushion of the seatand supported by the base of the seat, wherein said bladder isconstructed from a flexible material; b. a fluid contained by saidbladder, wherein said fluid is a liquid; c. a pressure sensoroperatively couipled to said bladder for generating a signal responsiveto the pressure of said fluid within said bladder; and d. a signalprocessor for measuring the weight of the occupant from said signal forgenerating a control signal for controlling the safety restraint systemresponsive to said weight measurement.
 15. A system for measuring theweight of an occupant in a vehicle seat and for controlling a safetyrestraint system responsive thereto as recited in claim 14, whereby saidfluid is a silicone based fluid.
 16. A system for measuring the weightof an occupant in a vehicle seat and for controlling a safety restraintsystem responsive thereto, comprising:a. a bladder mounted beneath thecushion of the seat and supported by the base of the seat, wherein saidbladder is constructed from a flexible material; b. a fluid contained bysaid bladder; c. a pressure sensor operatively coupled to said bladderfor generating a signal responsive to the pressure of said fluid withinsaid bladder, wherein said pressure sensor is responsive to the strainin the surface of said bladder; and d. a signal processor for measuringthe weight of the occupant from said signal for generating a controlsignal for controlling the safety restraint system responsive to saidweight measurement.
 17. A system for measuring the weight of an occupantin a vehicle seat and for controlling a safety restraint systemresponsive thereto, comprising:a. a bladder mounted beneath the cushionof the seat and supported by the base of the seat, wherein said bladderis constructed from a flexible material, b. a fluid contained by saidbladder; c. a pressure sensor operatively coupled to said bladder forgenerating a signal responsive to the pressure of said fluid within saidbladder, wherein said pressure sensor is internally integrated withinsaid bladder and d. a signal processor for measuring the weight of theoccupant from said signal for generating a control signal forcontrolling the safety restraint system responsive to said weightmeasurement.
 18. A system for measuring the weight of an occupant in avehicle seat and for controlling a safety restraint system responsivethereto, comprising:a. a bladder mounted beneath the cushion of the seatand supported by the base of the seat, wherein said bladder isconstructed from a flexible material; b. a fluid contained by saidbladder; c. a pressure sensor operatively coupled to said bladder forgenerating a signal responsive to the pressure of said fluid within saidbladder, wherein said pressure sensor is isolated from said fluid by thesurface of said bladder and d. a signal processor for measuring theweight of the occupant from said signal for generating a control signalfor controlling the safety restraint system responsive to said weightmeasurement.
 19. A system for measuring the weight of an occupant in avehicle seat and for controlling a safety restraint system responsivethereto as recited in claim 18, further comprising a restraint locatedon the inside of said bladder proximate said pressure sensor, wherebysaid restraint prevents said bladder from collapsing proximate saidpressure sensor.
 20. A system for controlling a safety restraint systemresponsive to the weight on a vehicle seat, comprising:a. a fluidcontaining means operatively coupled to the vehicle seat for generatinga pressure within said fluid responsive to the weight on the vehicleseat, wherein said fluid containing means is partially filled with saidfluid; b. a means for sensing the pressure of said fluid; and c. a meansfor controlling the safety restraint system responsive to the pressureof said fluid.
 21. A system for controlling a safety restraint systemresponsive to the weight on a vehicle seat as recited in claim 20further comprising a means for distributing the weight over said fluidcontaining means.
 22. A system for controlling a safety restraint systemresponsive to the weight on a vehicle seat, comprising:a. a fluidconfinement operatively coupled to the vehicle seat whereby the pressureof said fluid is responsive to the weight on the vehicle seat, whereinsaid fluid confinement is partially filled with said fluid; b. apressure sensor operatively coupled to said fluid confinement forgenerating a signal responsive to the pressure of said fluid; and c. asignal processor operatively coupled to said pressure sensor forcontrolling the safety restraint system responsive to said signal.
 23. Asystem for controlling a safety restraint system responsive to theweight on a vehicle seat as recited in claim 22 further comprising aload distributor whereby said load distributor distributes the weight onthe vehicle seat over at least a portion of the surface of said fluidconfinement.
 24. A method for controlling a safety restraint systemresponsive to the weight of an occupant in a vehicle seat, comprising:a.interposing a hydrostatic weight sensor in series with the load paththat supports the occupant in the vehicle seat, whereby said hydrostaticweight sensor is partially filled with a fluid and incorporates apressure sensor in fluid communication therewith, said pressure sensorgenerates a signal responsive to the pressure of said fluid, and saidpressure is responsive to the component of occupant weight applied bythe occupant on the vehicle seat to said hydrostatic weight sensor; andb. controlling the safety restraint system responsive to said signalresponsive to the pressure of said fluid.
 25. A method for controlling asafety restraint system responsive to the weight of an occupant in avehicle seat as recited in claim 24 further comprising the act ofdistributing said fluid within a plurality of zones in fluidcommunication with one another within said bladder.
 26. A method forcontrolling a safety restraint system responsive to the weight of anoccupant in a vehicle seat as recited in claim 25, further comprisingthe act of non-uniformly distributing said plurality of zones withinsaid bladder.
 27. A method for controlling a safety restraint systemresponsive to the weight of an occupant in a vehicle seat as recited inclaim 24, further comprising the act of forming at least one zone withinsaid bladder, whereby said at least one zone is not in fluidcommunication with said pressure sensor.
 28. A method for controlling asafety restraint system responsive to the weight of an occupant in avehicle seat as recited in claim 24 whereby said pressure sensor sensesthe pressure of said fluid from the outside of said bladder.
 29. Amethod for controlling a safety restraint system responsive to theweight of an occupant in a vehicle seat as recited in claim 28, furthercomprising the act of preventing said bladder from interfering with theoperation of said pressure sensor.
 30. A method for controlling a safetyrestraint system responsive to the weight of an occupant in a vehicleseat as recited in claim 24 wherein said safety restraint systemcomprises an air bag inflation system, and the act of controlling saidsafety restraint system comprises controlling the inflation rate of saidair bag inflation system.
 31. A method of manufacturing a hydrostaticoccupant weight sensor, comprising:a. securing first and second portionsof coated fabric to one another along a periphery, so as to form aconfinement, said confinement having an opening; b. securing said firstand second portions of said coated fabric to one another within saidperiphery so as to create a plurality of zones within said periphery influid communication with one another; c. operatively coupling a pressuresensor to said confinement; d. at least partially filling saidconfinement with a fluid through said opening in said confinement; ande. sealing said opening in said confinement.
 32. A method ofmanufacturing a hydrostatic occupant weight sensor as recited in claim31, wherein said plurality of zones are characterized by a plurality ofshapes.
 33. A method of manufacturing a hydrostatic occupant weightsensor as recited in claim 31, wherein, wherein said plurality of zonesare characterized by a plurality of sizes.
 34. A method of manufacturinga hydrostatic occupant weight sensor as recited in claim 31, whereinsaid pressure sensor is isolated from said fluid by said coated fabric.35. A method of manufacturing a hydrostatic occupant weight sensor asrecited in claim 31, further comprising the step of installing arestraint within said confinement to prevent said confinement frominterfering with the operation of said pressure sensor.
 36. A method ofmanufacturing a hydrostatic occupant weight sensor as recited in claim31, wherein the step of securing said first and second portions of saidcoated fabric comprises welding.
 37. A method of manufacturing ahydrostatic occupant weight sensor as recited in claim 31, wherein saidfluid is a silicone based fluid.
 38. A method of manufacturing ahydrostatic occupant weight sensor as recited in claim 31, wherein saidconfinement is partially filled with said fluid.